The present invention relates to electron holography apparatuses, and more particularly to an electron holography apparatus which can three-dimensionally measure the phase distribution of an electron beam precisely and rapidly.
FIG. 10 schematically shows a hologram formed by off-axis electron holography which uses an electron microscope provided with an electron biprism (that is, an electron beam splitter). The hologram consists of equally spaced fringes called carrier fringes. However, the phase of an electron beam is varied by the presence of a specimen, an electric field, or a magnetic field, and hence the portion of the interference fringe pattern 1 which corresponds to the above factors deviates from the carrier fringes. The deviation is proportional to a change in phase of the electron beam. Accordingly, a phase distribution map of the electron beam can be formed by detecting the deviation of the interference fringes from the carrier fringes.
Hence, the following method has been used to obtain the phase distribution map. That is, as described in U.S. Pat. No. 4,532,422, a basic, straight fringe pattern having the same spatial frequency as the carrier fringes of the hologram is superposed on the hologram to obtain visual equi-phase lines on the basis of the Moire principle. Thus, the phase distribution is expressed by equi-phase lines which are formed at a phase interval of 2.pi.. In order to improve the precision of the phase distribution measurement, it is required to form one or more equi-phase lines between adjacent Moire fringes by an appropriate interpolation method. However, it is very difficult to obtain an accurate phase distribution map in such a manner. This is because the brightness distribution in the interference fringe pattern is affected not only by the phase distribution of the electron beam but also by the amplitude distribution thereof, and thus the phase distribution does not necessarily correspond to the brightness distribution in the interference fringe pattern. Further, in this method, basic, straight fringes are left unremoved, in addition to the Moire fringes, and act as noise.
Another method of determining the phase distribution of an electron beam uses an optical interferometer, as shown in FIG. 1 of an article entitled "Interference Electron Microscopy by Means of Holography" by J. Endo et al. (Japanese Journal of Applied Physics, Vol. 18, No. 12, December, 1979, pages 2291 to 2294). In this method, the basic straight fringes do not appear, but the phase distribution is expressed by the equi-phase lines. Further, a method of measuring the phase distribution of an electron beam so that a phase difference less than 2.pi. can be detected, is known. As shown in FIG. 5 of the above-mentioned article and the explanation of FIG. 5, this method uses phase-difference-amplification techniques. However, the method requires not only a great deal of skill but also much labor and time, and the precision of measurement of phase distribution is limited to about 2.pi./50. This value is not satisfactory.
As mentioned above, the conventional methods can not satisfy three important requirements (that is, the high-precision measurement of phase distribution, the high-speed measurement of phase distribution, and the acquisition of three-dimensional information on phase distribution, that is, phase information capable of discriminating between lead and lag with respect to phase) at the same time.